The U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) is looking for ways to upgrade the Phosphorus Index, a simple management tool developed during the 1990s to gauge the risk of phosphorus losses from agricultural fields. In developing a national nutrient-management policy, NRCS allowed states to modify the original index—a matrix of source and transport factors that contribute to phosphorus loss—with inputs to account for local variations in soils, climate, management and water quality goals.

But this action resulted in widely different state-by-state phosphorus indices that often didn’t agree with each other on how to manage phosphorus. “We’d put in the same numbers and get different results, depending on the state,” says Agricultural Research Service soil scientist Peter Vadas, who works at the U.S. Dairy Forage Research Center in Madison, WI. Additionally, many of these indices were not tested against monitoring data to show that they gave reliable results, or they didn’t quantify phosphorus loss—such as losses in pounds per acre—which made them difficult to test against field data.

Vadas worked with colleagues to develop the Annual Phosphorus Loss Estimator (APLE), a user-friendly spreadsheet program that predicts field-scale phosphorus losses in runoff for a whole year.

The scientists developed APLE so it would be as easy to use as earlier phosphorus indices. It could be used in many different states to quantify field-scale phosphorus loss and soil phosphorus changes over 10 years for a given set of runoff, erosion and management conditions. They included information and data that has been established for decades, as well as important innovations, especially for phosphorus loss from manures and fertilizers.

The scientists started out by programming APLE with data from 21 field studies that measured the impact of surface-applied manure and fertilizer on phosphorus loss. The 21 studies represented a wide variety of field sizes, crop and manure management conditions, manure types and geographic locations across the United States.

Then the scientists expanded APLE to include dissolved phosphorus loss from soil and sediment phosphorus loss from erosion. They tested APLE with field data from another 28 studies that monitored phosphorus loss in runoff from fields for at least a year.

The team showed that APLE could reliably quantify phosphorus losses in runoff for many different situations and could produce more reliable estimates than some existing phosphorus indices. From there, Vadas and his colleagues again expanded APLE to simulate changes in soil phosphorus over 10 years. This application would be important for farmers concerned about accumulating too much soil phosphorus or wondering how long it might take to reduce the phosphorus that had already accumulated.

The team compared APLE soil phosphorus simulations with field results from 25 studies that monitored changes in soil phosphorus from one to 29 years.

Most recently, the scientists have been using APLE to improve the assessment of phosphorus losses from agricultural fields and whole farms. For instance one project demonstrated that APLE can be used to rapidly and easily develop weighting factors for determining the relative contribution of phosphorus losses from different sources, such as soil, manure or erosion. In collaboration with scientists from the University of Wisconsin in Madison and Platteville, Vadas has been adapting APLE to simulate phosphorus losses from pastures grazed by beef and dairy cattle and from barnyards and exercise lots on cattle farms. With these improvements, APLE can be used to develop whole-farm estimates of phosphorus losses and the most effective strategies for reducing phosphorus losses from cattle farms. These practices could include barnyard improvements for capturing discharge, soil conservation practices that reduce erosion, or manure application practices that reduce exposure to runoff water.

“APLE has all of the current science, and it shows that we do have good equations to predict phosphorus loss,” Vadas says. “We have decades of research on soil phosphorus processes, but we needed to find a way to integrate this information so that people can make sense of it. Now we have a good model that can expand on different scenarios very quickly and efficiently and help producers and policymakers make informed decisions about managing phosphorus.”